Mr. Issa, California Republican, and Rep. Jim Jordan, Ohio Republican, are leading the House Oversight and Government Reform Committee’s IRS inquiry. They also said the FBI is refusing to turn over any documents related to its own investigation into the IRS, which began in the days after an auditor’s report revealed the tax agency had improperly targeted tea party groups for special scrutiny.

Six months after it began, the FBI’s investigation has resulted in no release of information. The congressmen said the FBI even rescinded an offer for an in-person briefing with the assistant director in charge of the investigation. The reversal, after the FBI consulted with the Justice Department, suggests political meddling, the two investigators said.

“The department’s tactics have impeded a congressional investigation and interfered with the committee’s access to documents and information. Obstructing a congressional investigation is a crime,” Mr. Issa and Mr. Jordan said in their letter to Mr. Comey, warning that if they don’t get the information they will use “compulsory” means.

The internal auditor for the IRS reported in May that the agency asked inappropriately probing questions and delayed conservative groups’ applications — in some cases for three years.

After the revelations, the IRS promised to clean up its operations, and Attorney General Eric H. Holder Jr. ordered the FBI to open a criminal investigation.

At the same time, however, one of the conservative activists whose groups were targeted came forward to say that she was assailed by both the IRS and the FBI, as well as other federal agencies.